22.6: Carbohydrates and lipids provide bodies with energy and more.

Reading these words requires carbohydrates in your body, obtained from your diet, to fuel your brain—and your muscles as well, as they help you sit up and hold your book. Food fat, too, a subset of the group of macromolecular nutrients called lipids (see Sections 2-12–2-14), provides energy for your body’s functioning and gives food delicious flavor (while contributing to weight gain and heart disease if consumed in too large a quantity). In this section, we evaluate the carbohydrates and lipids. These two crucial nutrients supply nearly all the energy that fuels your daily activities.

Carbohydrates: Fuel for Living Machines What are carbohydrates, and what is their role in the diet? Carbohydrates are the primary fuel on which animal bodies run. In humans, nearly all of the energy used by our brain every day comes from the simple carbohydrate glucose. As we saw in Chapter 2, all carbohydrates are made primarily from carbon, hydrogen, and oxygen. When the bonds between these atoms are broken, energy is released that can be captured by the body and used to fuel movement, growth, and all the other cellular activities that require energy.

We get the majority of our dietary carbohydrates from fruits, vegetables, and grains (FIGURE 22-9). And, as with proteins, the breakdown of carbohydrates for energy generates 4 kilocalories per gram.

Are all carbohydrates the same, or do they vary in important ways? Although, structurally, all carbohydrates are variations on a simple theme—molecules formed from carbon, hydrogen, and oxygen in the approximate proportions of CH₂O (see Figure 2-20)—they vary dramatically in their complexity and break down in the body at different rates (FIGURE 22-10). Animals consume carbohydrates in the form of simple sugars (monosaccharides), disaccharides and digestible complex sugars (starch and glycogen), and indigestible complex sugars (fiber).

Simple sugars. These include glucose and fructose. They are linear or ring structures with three to seven carbon atoms. Animals can break them down directly through the steps of glycolysis (see Section 4-13), rapidly releasing the stored energy from the bonds of the sugar.

Disaccharides and digestible complex sugars. Multiple simple sugars can bond together to form complex but digestible molecules. Disaccharides, such as sucrose (table sugar), are just two simple sugars joined together. Complex sugars, such as starch and glycogen, are large molecules that may consist of hundreds or thousands of glucose molecules connected in dense, branching patterns. For an animal to have access to the energy stored in the bonds of the individual simple sugars, it must first break the bonds that link those sugars together. As these bonds are broken, simple sugars become available for the energy-releasing reactions of glycolysis.

Fiber. This is a complex carbohydrate, such as cellulose, that forms the structural parts of plants. Fiber differs from starch and other digestible complex sugars by having a different bond connecting the simple sugars together. In humans, this bond cannot be broken by any digestive enzyme, making fiber indigestible. As we’ll see later in this chapter, although fiber isn’t broken down for energy or to supply other molecules used by the body, it still plays an important role in digestion and is necessary in the diet to maintain health.

How and where do humans store carbohydrates? Carbohydrates in our body are stored mostly as glycogen (see Section 2-9) in liver and muscle cells. At any given time, we can store only about one day’s worth of energy. After we start exercising, or when our bodies need energy for an activity, a signal is sent that causes the release of enzymes that break the bonds holding together the highly branched glycogen. This glycogen breakdown produces a flood of glucose into the bloodstream and in the muscles where the energy is needed.

Large amounts of water are bound to stored glycogen: 4 pounds of water for every pound of glycogen. Consequently, as glycogen in your liver and muscles is used, the water bound to it is released from the tissue and lost as urine. This is why, as stores of glycogen are depleted in the initial stages of a weight-reducing diet, there is a dramatic initial weight loss from the loss of water that was bound to the glycogen. As your body starts utilizing stored fat, the rate of weight loss slows considerably.

Fats: Long-Term Energy Storage Experts What are fats, and what is their role in the diet? Fats in the animal diet, described in detail in Chapter 2, function primarily as a dense source of energy that can be efficiently stored in the body (FIGURE 22-11). The average person has about four or five weeks’ worth of stored energy in the form of fat. Compared with carbohydrates or proteins, a given amount of fat contains more than twice as much stored energy: 1 gram of fat produces 9 kilocalories. Another feature that makes fats particularly efficient as energy-storage molecules is that, because they are hydrophobic, fats are stored without binding to water.

Because fats are poor conductors of heat, fats stored in a layer just beneath the skin can help to keep the body warm. Penguins and walruses, for example, can maintain relatively high body temperatures despite living in very cold habitats, because of their thick layer of insulating fat.

Here’s a perplexing fact: although the total amount of energy in a gram of fat is greater than that found in carbohydrates or proteins, fat isn’t the optimum nutrient for most situations. When it comes to exercise, for example, muscle cells need quick access to energy. The problem is that, in muscles, fat burns very slowly for energy. Remember from Chapter 4 that the universal source of chemical energy in the body is ATP. This means that the energy in fat or carbohydrate or protein must be captured as ATP before it is of use to muscle cells. It turns out to be much easier for the body to break down muscle glycogen and blood glucose to make ATP than to break down fat. In fact, the rate of ATP synthesis from carbohydrates is about double the rate from fats.

Are all fats the same nutritionally, or do they vary in important ways? In our diet, fats usually come in the form of fatty acids, long chains of carbon atoms with hydrogens attached. With proteins, we saw that there are essential and non-essential amino acids. Similarly, with dietary fats, there are essential and non-essential fatty acids. The essential fatty acids—including linoleic acid, from the omega-6 family of fatty acids—are those that humans cannot produce and must be consumed. Linoleic acid is essential as a building block for signaling molecules, such as some hormones; a deficiency can lead to infertility and difficulty lactating. Another essential fatty acid, important in a variety of metabolic processes, is linolenic acid, from the omega-3 family of fatty acids. Linolenic acid is used by the body to make fatty acids that are essential for normal growth and development, especially in the eyes and brain.

Another important distinction among dietary fats is between saturated and unsaturated fats (FIGURE 22-12). If each carbon within the chain is bonded to two hydrogen atoms, the molecule carries the maximum number of hydrogen atoms and is said to be saturated. (For a refresher, see Figure 2-31.) Conversely, if some of the carbons are bound to only a single hydrogen, the fatty acid is unsaturated.

When saturated, fatty acids are very straight and the fat molecules can be packed together tightly. This causes saturated fats to be solid at room temperature, like butter. When unsaturated, the fatty acids have kinks in the hydrocarbon tail and cannot be packed together as tightly. Consequently, unsaturated fats do not solidify so easily and tend to be liquid at room temperature, like vegetable oil. Because unsaturated fatty acids can accept one or more hydrogen atoms, they are a bit less stable and more reactive—that is, they will take part in a greater variety of chemical reactions—than saturated fatty acids, making them less likely to be stored as body fat.

Trans fats have been in the news because of their tendency when consumed to raise levels of low-density lipoprotein cholesterol, increasing the risk of coronary heart disease. Trans fat is made when hydrogen is added to vegetable oil—a process called hydrogenation (see Section 2-13 for a review of trans fat chemistry). Trans fat is often found in some of the same foods as saturated fat, such as vegetable shortenings, crackers, candies, cookies, snack foods, fried foods, baked goods, and other foods made with partially hydrogenated vegetable oils. The American Heart Association recommends that these fats be minimized in the diet.

How and where do we store fats? If a human consumes more calories than he or she burns, most of the excess (regardless of whether these calories were consumed as carbohydrate, fat, or protein) gets converted to fat and stored in fat cells distributed throughout the body. A pound of body fat holds 3,600 kilocalories worth of energy. It is like a savings account for an uncertain future.

Given that our bodies can convert excess calories into body fat, regardless of whether the calories were initially ingested as carbohydrates or proteins, why is it still an effective weight-management strategy to minimize dietary fat intake? The answer rests in the number of chemical conversions carbohydrates and proteins must undergo to become body fat. In the case of carbohydrates, complex sugars must first be broken down into simple sugars—a process that requires energy. Then the simple sugars must be broken down and the fragments reassembled as fatty acids—another process that requires energy. By the time those fatty acids can be stored as body fat, much of the original energy stored in the carbohydrates has been lost in fueling all the chemical conversions (FIGURE 22-13). And for someone trying to minimize body fat, that’s good news. Storing protein as body fat also requires several energetically expensive conversions that reduce the ultimate quantity of fat stored.